LWD or drilling derived logging depths can be inaccurate and therefore unreliable for well-to-well correlations, correlations to offset well data, and formation thickness determinations. These inaccuracies generally originate from the fact that the LWD depth measurement is purposely made to equal the driller's depth. The actual depth of the measurements downhole is rarely equal to this static pipe measurement for the primary reason that while drilling, the drillpipe is subject to an environment that is not representative of that at the surface. Depths differ because of stress variations along the drillstring due to dynamically changing friction factors, varying weight applied to the drillbit, and temperature changes with depth. We demonstrate that it is sufficient to dynamically correct the LWD depth for drillstring weight, downhole friction, weight on bit, thermal expansion, residual rig heave and tide. The technique for quantifying the friction factors is based on the industry-accepted torque and drag model. Calibration of this model can be achieved using four parameters per bit-run. The method also provides uncertainty estimation for the depth correction. The technique is implemented as a software application and underwent extensive field-testing with successful results. Examples of the application are presented.
展开▼
